1. Field of the Invention
The present invention relates to a controller and a control method for use in a radio station in the environment where several radio stations share a common frequency band.
2. Description of the Related Art
In current radio communications, different dedicated frequency bands are often assigned to radio communication systems in order to avoid mutual interference between the radio communication systems. For more efficient utilization of limited frequency resources, however, it has been recently discussed that several radio communication systems use a common frequency band. In order to share such a common frequency band, multiple radio stations or multiple radio communication systems must comprehend frequency usage states each other and control transmissions to avoid occurrence of interference. Basically, there are two approaches for the transmission control. One of the approaches relates to distributed control where individual radio stations monitor frequency usage states around themselves and autonomously determine availability of transmissions. The other approach relates to centralized control where a control station provided for uniformly managing frequency usage states of the radio stations determines availability of respective transmissions by the radio stations.
In one conventional technique, a common frequency band is efficiently reused over different cells in a radio communication system. In normal cellular systems, one scheme referred to as “inter-cell frequency reuse” may be utilized. In the inter-cell frequency reuse scheme, different frequency bands are used in adjacent cells to avoid inter-cell interference. A document “3GPP.R1-060670” proposes an interference coordination scheme where frequency bands to be assigned for radio stations residing in cell boundaries are predetermined such that the frequency bands for use in the cell boundaries can be assigned for adjacent cells differently from each other for avoidance of the inter-cell interference. Also, Japan Laid-Open Patent Publication No. 2007-258844 proposes one technique for reducing influence of interference on the radio stations residing in cell boundaries by grouping the radio stations based on CQI (Channel Quality Indicator) information reported from the radio stations and assigning resource blocks corresponding to different transmission power levels for the radio station groups. In addition, Japan Laid-Open Patent Publication No. 2007-258844 proposes another technique for preventing occurrence of the interference due to usage of a common resource block around cell boundaries by causing base stations to measure interference power caused from adjacent cells and adjusting power allocated to the common resource block for cells having significant interference.
Furthermore, Japan Laid-Open Patent Publication No. 2008-278273 discloses that assignment of interference area resources, that is, resources where interference may be likely to arise, is inhibited based on resource assignment information shared among radio stations in a radio communication system. The disclosed scheme may relate to the above-mentioned centralized control approach. Frequency usage state information is shared among the neighboring radio stations beforehand, and a control station assigns resources for the individual radio stations based on collected information pieces to avoid occurrence of interference.
On the other hand, a CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) scheme is known as one technique for utilizing a common frequency band under distributed control. The CSMA/CA are described in detail in a document “Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications, ANSI/IEEE Std 802.11, 1999 Edition”, for example. In the CSMA/CA scheme for use in wireless LAN systems typically complying with the IEEE 802.11 standard, reception level is measured prior to data transmissions in order to determine availability of the data transmissions. If the CSMA/CA scheme is utilized in environments where different radio communication systems share a common frequency band, individual radio stations determine availability of transmissions based on respective radio communication environment information pieces separately collected by the radio stations. Thus, even if the collected radio communication environment information pieces are not shared among the radio communication systems, interference can be avoided by the individual radio stations activating the transmissions only upon failure of signal detection.
Also, Japan Laid-Open Patent Publication No. 2006-222665 discloses some technique for avoiding interference by radio stations detecting the presence of signals through calculation of features for signal cyclostationarity and activating transmissions only without detection of the signals. According to this technique, even when different radio communication systems use a common frequency band, the radio stations can detect the presence of signals being used around themselves and fulfill transmissions if it is determined that no interference may arise.
In the case where different radio communication systems share a common frequency band, there is a likelihood that information may not be necessarily exchanged between radio stations. Under the centralized control over radio resources, if the information cannot be appropriately transmitted, there is a risk that signal quality may be significantly degraded. Specifically, according to the conventional scheme as disclosed in 3GPP. R1-060670, although a common frequency band is shared among user apparatuses distinguished from each other by means of spread codes, radio stations belonging to different radio communication systems cannot always use the spread codes or other information, which means that they may not be able to successfully avoid occurrence of interference. Also, according to the conventional interference reduction scheme through interference coordination of radio stations residing in cell boundaries, there is a likelihood that precise cell design may be impossible for some supposed frequency sharing environments, which may cause some areas having significant degradation of characteristics due to interference. Furthermore, according to the conventional scheme as disclosed in Japan Laid-Open Patent Publication No. 2007-258844, base stations group mobile stations based on CQI information reported from the mobile stations and assign resources for individual groups. Since radio stations in different radio communication systems cannot share the CQI information through exchanges of radio signals, it may be difficult to fulfill the grouping and appropriate resource assignment. Also in the scheme as disclosed in Japan Laid-Open Patent Publication No. 2008-278273, resource assignment information cannot be similarly shared in the suggested environments, resulting in significant characteristics degradation. As stated above, the centralized control scheme can only be implemented with high complexity in that a larger amount of information is required. In addition, the centralized control scheme may degrade the characteristics significantly under some environments where it is difficult to share information among different radio communication systems, for example, under the environment where a common frequency band is shared among the different radio communication systems.
In contrast, in the distributed control using the conventional CSMA/CA scheme or the scheme as proposed in Japan Laid-Open Patent Publication No. 2006-222665, radio stations cannot always make appropriate determination on transmission availability. For example, although communications are ongoing near a certain radio station and accordingly that radio station should defer transmissions, the radio station may determine that some radio resources are available from some reasons by itself and use these radio resources, resulting in occurrence of interference. On the other hand, although no neighboring radio station uses a radio resource, a radio station may determine that that radio resource is being used due to erroneous detection and defer transmissions. In this case, interference may not arise, but frequency utilization efficiency may be reduced. As stated above, according to the distributed control scheme, interference can be avoided even in situations where information is insufficiently shared compared to the centralized control and can be implemented with low complexity. However, the distributed control scheme could achieve less beneficial interference avoidance effect and/or lower frequency utilization efficiency than the centralized control scheme.